In so called closed loop WCDMA systems, instructions to a UE regarding the UE's uplink transmission are transmitted to the UE from the NodeB, although the instructions may in some cases originate from the RNC.
The instructions for uplink transmissions may relate to, for example, the number of streams to be used in uplink MIMO transmissions or uplink beam forming transmissions. The beam forming case will be described briefly below, which is for the sake of clarity only, and is not intended to exclude the MIMO case.
Beam forming in transmissions from a UE can be seen as multiplying the transmit signal at each antenna or antenna port (a notion which will be explained in more detail later in this text) with a certain weight factor, where the weight factor can be either complex or non-complex. For a UE with more than one antenna or antenna port, this can be seen as a pre-coding vector comprising the pre-coding factors to be used by the UE, where the size of the pre-coding vector corresponds to the number of antennas or antenna ports.
In closed-loop beam forming, the desired pre-coding vector is determined by the NodeB and then signaled to the UE. Usually, a pre-coding vector is signaled to a UE by means of signaling a code word from a code book which comprises a number of such code words, where each code word corresponds to a certain pre-coding vector in the code book.
With such downlink signaling to the UE, it will be realized that in general, there will be a trade-off between signalling overhead and UE performance. The higher granularity that is available for the pre-coding weights (i.e. the larger the codebook), the more efficiently the NodeB can adapt to the effective channel. On the other hand, a large codebook that consists of many code words will result in an increased amount of downlink feedback overhead, since, in general, the NodeB needs to be able to signal all unique code words to the UE, and the number of bits required to signal all code words in a codebook of size K is log2 K.
In order to keep track of the fast variations of the wireless channel (e.g. fast fading) the pre-coding weights need to be updated frequently, typically on a per slot basis. A WCDMA channel with a structure that is suitable for carrying the pre-coding weight selection instructions, i.e. the code words mentioned above, is the fractional DPCH, the F-DPCH channel.